Psilocybin Builds New Brain Connections in Mice Within 24 Hours

Within 24 hours of a dose of psilocybin, mice grew a large number of new connection points on their brain cells, and many of them matured unusually fast, according to a study from the University of California, Santa Cruz. The work was published Oct. 15 in Nature Neuroscience.

The team also found that the rewiring and a behavior used as a sign of psychedelic activity in rodents run through separate pathways. In some mice, the brain changes happened without that behavior. All of the experiments were done in mice, so the results have not been shown in people.

How the team watched connections form in living mice

The researchers used two-photon microscopy, an imaging method that can follow very small structures inside living tissue. With it they tracked individual connection sites on neurons in the same mice for several weeks after treatment.

They focused on dendritic spines, small bumps on neurons that receive signals from neighboring cells and help form synapses, the junctions where nerve cells communicate. New spines usually start out short and “stubby,” and many vanish before they mature. After psilocybin, a much larger share developed into the mushroom-shaped form that tends to be more stable and carries more receptors for chemical signals.

Psilocybin also removed some older connections. In healthy mice, spine numbers jumped at first and then drifted back toward their earlier level over the next three weeks, partly because the brain cleared out connections that existed before the dose. The authors describe this as reorganization of existing circuits, with the overall count kept in balance.

Stressed brains reacted differently

To model long-term stress, the team used an unpredictable mild stress model, in which mice face a changing series of low-level stressors. The model produces changes that resemble some features of human depression, and chronic stress is known to cost the brain synaptic connections.

In these stressed mice, psilocybin set off an especially strong burst of new spines. It also blocked the unusually high rate of connection loss that comes with chronic stress. The researchers say this points to the drug helping preserve and restore circuits damaged by stress, though that interpretation comes from animal data.

“These findings challenge the prevailing view that psilocybin simply increases neural connectivity,” said senior author Yi Zuo, a professor of molecular, cell, and developmental biology at UC Santa Cruz.

Separating the rewiring from the head twitch

Psilocybin and other classic psychedelics act on serotonin 2A receptors (5-HT2ARs), proteins on cell surfaces that respond to chemical signals. The team looked at Layer 5 pyramidal neurons in the cerebral cortex, cells that carry information from the cortex to other brain regions and hold many of these receptors.

Using genetically modified mice, the researchers controlled whether those specific neurons had the receptors. When the receptors were restored only in Layer 5 pyramidal neurons, psilocybin still triggered new synapses and helped stabilize them, but the mice did not show the head twitch response that scientists use to gauge psychedelic activity in rodents. When the receptors were removed from those neurons alone, the structural growth disappeared while the head twitch remained.

According to the study, receptor signaling in these neurons is both necessary and sufficient for the structural changes, and it is not needed for the head twitch. A head twitch in a mouse is a stand-in measure, so the result does not show directly that hallucinations in people could be avoided.

What it means for psychedelic research

Researchers sometimes call psychedelics “psychoplastogens” because a single dose can stimulate neuroplasticity, the brain’s ability to form, strengthen, reorganize or remove connections. Depression can involve disrupted circuits for mood and emotion, which is why this property draws interest.

The UC Santa Cruz team says the findings could someday help scientists design drugs that produce some of psilocybin’s effects on brain structure without hallucinations, with possible uses in depression and post-traumatic stress disorder (PTSD). No such treatment exists yet, and the work has not been tested in humans.

The study arrives as U.S. regulators look more closely at psychedelics. The Food and Drug Administration has given Breakthrough Therapy designations to psilocybin and MDMA, and in September 2026 it held a public hearing on possible future therapeutic use of psychedelic drugs. Speakers there discussed who should be qualified to give such treatments, how to manage risks through Risk Evaluation and Mitigation Strategy (REMS) programs, and how future clinical trials should be designed.

Lead author Jacob Baker, a Ph.D. student in Zuo’s lab, said the new connections can join existing circuits and last over time.

Source: ScienceDaily